WHY are there so many sects in Christendom? Their number calls forth the derision of unbelievers, and causes grief to the devout.
When the apostles taught, Christianity began with a single "sect", which was said to be "everywhere spoken against" (Acts 28:22). If the teachings held by its members had remained exactly as they were given by the first teachers, there is at least a chance that it would have continued to be one "sect". But suppose that fresh doctrines are introduced, old words are used to clothe new ideas, and an alien philosophy is combined with the original teaching? Is it not clear that there will be a strain between the new beliefs and the old? The way will be opened for all kinds of combinations of ideas, and for any amount of ingenuity to invent new theories to reconcile things which are in conflict. And as a result, different groups will drift further and further apart until they split into separate churches in perpetual disagreement with one another.
Put broadly and simply, that is very much what happened to Christianity. The changes were coming even in the apostles' days, for we find Paul strenuously opposing ideas which would have made the resurrection of the dead unnecessary (1 Cor. 15:12-20). At the end of his life, Paul is still combating those "who concerning the truth have erred, saying that the resurrection is past already" (2 Tim. 2:18). There is a strong probability, to put it no higher, that the only teaching which could displace the belief in future resurrection is the Greek doctrine that the soul is immortal, and therefore needs no raising from the grave. To Paul this was a matter of acute concern, for its effects were disastrous. It had "overthrown the faith of some", and as to the future, he saw that it would "eat like a gangrene" (v. 17, R.V.).
In some of his earliest letters Paul had said there would come a "falling away", and even that "the mystery of iniquity doth already work" (2 Thess. 2: 3, 7). He told the Ephesians, "Of your own selves shall men arise, speaking perverse things" (Acts 20:30). In both his letters to Timothy he writes of times when "some shall depart from the faith" (1 Tim. 4:1), and when "they will not endure sound doctrine", but will turn away their ears from the truth, and turn aside unto fables (2 Tim. 4:3- 4). Having "itching ears", he says, they will "heap to themselves teachers" according to their own desires.
Nor is Paul alone in this. Peter says, "There shall be false teachers among you, who shall privily bring in damnable heresies" (2 Peter 2:1). The letters of John, written in his extreme old age towards the end of the first century, are very largely devoted to combating teaching which was already prevalent. "Many false prophets (or teachers) are gone out into the world", he says (1 John 4:1). "Many deceivers are entered into the world" (2 John v. 7). He urges believers to put to the test those who come to them as teachers (or in his own idiom, to "try the spirits"); and the only standard by which they can be tested is the Word which God has already given through His Holy Spirit -- that is, Scripture.
Even in the apostles' day, when only the writings we now call the Old Testament could be appealed to, the Jews at Berea were called "more noble" because they "searched the scriptures daily" to see whether the things they were told by Paul and Silas "were so" (Acts 17:11). And when in earlier times Jews had sought to communicate with the dead by practices like those of modern Spiritualism, Isaiah had pointed to the only true source of inquiry: "Should not a people seek unto their God? ... To the law and to the testimony: if they speak not according to this word, it is because there is no light in them" (Isa. 8:19-20). "Prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Spirit" (2 Pet. 1:21).
There are many sects because Christianity as a whole has gone astray from the teaching of Christ. The greatest need of today is to return to the early faith: and the only way to return is by the study of the Scriptures.